Monomethylated Tröger's-base-containing polyimidazolinium salt as an efficient catalyst for cycloaddition of CO2 to epoxides

In this work, the model reaction of benzaldehyde and ammonium chloride has been studied by detailed product analysis and characterization. Subsequently, amarine and iso-amarine hydrochlorides (1, 2), dimethylated amarine and iso-amarine ionic salts (3, 4), together with monomethylated Tröger's-base ionic salt (6) have been separately synthesized and characterized. In addition, their catalytic performance towards the cycloaddition of CO2 to epichlorohydrin is compared under identical conditions. It is observed that their catalytic activity follows the order of 3 ≈ 4 > 6 > 1 ≈ 2. Inspired by the highly catalytic activity of 3, 4 and 6, a novel monomethylated Tröger's-base-containing polyimidazolinium salt (namely PMDD-IL) has been synthesized by one-pot reaction of Tröger's-base-containing dialdehyde monomer (MDD) with ammonium chloride followed by treatment with excess iodomethane in the presence of sodium hydride. The as-synthesized PMDD-IL network exhibits highly catalytic activity towards the cycloaddition of CO2 to epoxides, providing nearly quantitative selectivity (99%) and conversion (99%) over a wide substrate scope of epoxides at atmospheric CO2 pressure (epichlorohydrin, 100 °C/10 h; epibromohydrin, 100 °C/12 h; glycidyl phenyl ether, 100 °C/24 h; styrene oxide, 100 °C/24 h; and 2-butyloxirane, 100 °C/24 h). The excellent catalytic activity of PMDD-IL network can be ascribed to the high nucleophilicity of iodine anion along with the strong interactions between PMDD-IL network and CO2 molecules.